Here is the line from repeater_1tick.json for the bottom face of the repeater:

"down":  { "uv": [ 0,  0, 16, 16 ], "texture": "#slab", "cullface": "down" }

except...the bottom face is not rendered. The screenshot shows a floating repeater with no blocks touching, yet the bottom face is culled. Removing culling for the face makes it render, but by default it does not, as if the repeater were always against a solid block. Removing the cullface property like so will make the bottom face render.

"down":  { "uv": [ 0,  0, 16, 16 ], "texture": "#slab" }

However, then the bottom face will never be culled, even when next to a solid block

If a repeater uses the cube model, both the top and bottom faces will be culled.

Relates to MC-50734